1. Field of the Invention
The present invention relates to a ball screw applied for linear transmission, and more particularly to a return structure for a ball screw with a chain type rolling assembly.
2. Description of the Prior Art
As for precision machineries, in order to control the feed rate precisely and with low friction coefficient, ball screws are used for precision transmission. Such a ball screw is normally provided with a screw, a nut and plural balls between the screw and the nut. The balls roll circularly within the nut through return assemblies provided on the nut. Referring to FIG. 9, a nut 51 is moveably mounted on a screw 50 and provided with a return assembly 52 at each of two opposite ends thereof. The screw 50, the nut 51 and the return assemblies 52 define a circulation path 54 for endless circulation of a rolling assembly 55. In order to avoid undesired rotation of the rolling assembly 55, the circulation path 54 is defined with guiding grooves 541. The rolling assembly 55 is provided with two linking portions 551 at two opposite sides thereof for linking plural rolling elements 550 together. The linking portions 551 slide along the guiding grooves 541 of the circulation path 54, and the guiding grooves 541 at the two sides of the respective return assemblies 52 are disposed along the arcs defined by the smallest radius and the biggest radius of a return portion 520, so that when the rolling assembly 55 passes through the return portion 520, the linking portions 551 at the two sides of the rolling assembly 55 will slide along the guiding grooves at the two sides of the return assembly 52 and bend therein, the direction in which the linking portions 551 bend is obviously vertical to the bendable direction of the linking portions 551, thus causing serious damage to the linking portions 551.
After entering one of the return assemblies 52, the linking portions 551 will move along the guiding grooves 541 at the two sides of the one of the return assemblies 52 and pass through the right-angled return portion 520 of the one of the return assemblies 52 and then pass through the right-angled return portion 520 of the other of the return assemblies 52 and finally return between the screw 50 and the nut 51. When passing the right-angled return portion 520, the linking portions 551 at the two sides of the rolling assembly 50 are located within the arcs defined by the smallest radius and biggest radius of the return portion 520, namely one of the linking portions 551 is located within the arc defined by the biggest radius of the return portion 520 and subject to an improper stretching force, and the other of the linking portions 551 is located within the arc defined by the smallest radius of the return portion 520 and subject to an improper extrusion force. Due to the design of the linking portions 551 of the rolling assembly 55, the bendable direction of the rolling assembly 55 is located at a side where no linking portions 551 are disposed. The direction in which the guiding grooves 541 at the two sides of each of the return assemblies 52 guide the linking portions to bend is located on the linking portions at the two sides of the rolling assembly 55, so that when passing the respective return portions 520, one of the linking portions 551 will be improperly stretched, and meanwhile the other of the linking portions 551 will be extruded. As a result, the linking portions 551 of the rolling assembly 55 are more likely to rupture due to being improperly stretched or extruded.
When the linking portions 551 at the two sides of the rolling assembly 55 are improperly stretched and extruded synchronously within the return portion 520, they slide at very high speed, and the rolling elements 550 which change the moving direction quickly when entering the return portion 520 are likely to collide with the inner surface of the return assembly 52 to cause vibration. Therefore, when passing the return portion 520, the rolling assembly 55 will be affected by the vibration of the rolling elements 550 or the pulling force between the rolling elements 550, in addition, the rolling assembly 50 is more likely to rupture immediately since the linking portions 551 at the two sides of the rolling assembly 55 are improperly or excessively stretched or extruded. Moreover, since the linking portions 551 at the two sides of the rolling assembly 55 are stretched or extruded synchronously, the rolling elements 550 will deviate from its original path, thus causing undesired friction which will make the circulation of the rolling assembly unsmooth. Additionally, the improper collision will cause noise.
Another conventional chain-type ball screw is shown in FIGS. 10-12, wherein the tubular return member 3 is mounted on the radial outer surface of the nut 2 and comprises two engaging members 15 and a connecting member 16 connecting the two engaging members 15. In the tubular member 3 are formed two guiding grooves 17 for guiding the movement of the linking portion of the chain of the rolling assembly (not shown). The engaging members 15 are elliptical and used to connect the helical rolling groove (not shown) of the nut 2. The abovementioned problems that “the linking portions at the two sides of the rolling assembly are stretched or extruded synchronously, the rolling elements will deviate from its original path, causing undesired friction which will lead to unsmooth circulation of the rolling assembly” is still not solved by this conventional return member 3 due to the fact that the guiding grooves 17 in the engaging members 15 are twisted a predetermined angle though, the guiding grooves 17 in the connecting member 16 are not twisted, and importantly, the guiding groove 17 in the connecting member 16 are just located at the right-angled return portion of the circulation path where the linking portion of the chain of the rolling assembly will be subjected to an improper extrusion force and stretching force, respectively. On top of that, the nut 2 must be elliptical in order to form a flat mounting surface for mounting the return member 3, further, the return member 3 is also elliptical and mounted on the radial outer surface of the nut instead of inside the nut 2, which requires more installation space and the return member 3 is more likely to be damaged by external forces.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.